Exposure of a solid surface with interfacing materials is known to lead to “wear”, i.e. loss of material from the surface. Major types of wear include abrasion, erosion and corrosion. In the contact between two moving bodies there is usually a certain degree of friction. Excessive friction may lead to power losses and heating of the contact which is often undesired. The yearly worldwide economic losses and technical problems due to friction and wear are significant. Lubrication is a common way of minimizing and/or overcoming the problems associated with friction and wear. A large number of lubricant compositions for various purposes exist on the market.
A lubricant is often composed of a base fluid, traditionally a mineral oil and an additive package. The additive package may contain one or several chemical compounds designed to increase the performance of the lubricant. Examples of different kinds of additives include viscosity modifiers, detergents, dispersants, anti-wear additives, extreme pressure additives, friction modifiers, anti-corrosion agents and antioxidants.
Traditionally, lubricants for hydraulic and lubrication purposes are based on mineral-based oils. Mineral-based oils, including mixtures of alkanes in the C15 to C40 range from a non-vegetable source such as a distillate of petroleum, polyalphaolefin (PAO) and so on have good lubrication properties and contribute to reduction of friction and wear. However, lubricants from these mineral-based oils are not biodegradable and therefore remain in the eco-system for a long time, when they are released into the environment. In addition, these mineral-based oils are often toxic. For instance, contamination of the environment with mineral oil may make the soil unusable, water unfit for irrigation and sewage work inoperable. Even small amounts of mineral oil may have large and detrimental effects on the environment. For instance, one liter of oil released into the environment may cover an area of the size of a football pitch and contaminate as much as one million liters of water. Every year huge sums are spent by society and companies on cleaning and on measures to remedy damages due to leakage of mineral oil-based lubricants.
Many of the additives used in lubricants also have detrimental effects on the environment e.g. by showing low biodegradability or containing elements, such as sulfur or heavy metals, that are undesirable to release into the environment.
Economic and environmental concerns have therefore prompted the development of lubricants being less toxic or non-toxic as well as biodegradable. These so-called green lubricants are environmentally friendly, and are often synthetic ester-based lubricants or lubricants based on vegetable oil. For instance, the environmentally based lubricant may be based on rapeseed oil or sunflower oil. Often, the environmentally friendly lubricants are called green lubricants or environmentally considerate lubricants (ECLs). Compared to traditional mineral oil-based lubricants, these lubricants are much less harmful due to their lack of toxicity or very limited toxicity, and their relatively quick degradation into non-toxic residues in nature.
However, the performance and cost of environmentally friendly lubricants based on vegetable oil are often not as beneficial as the mineral-based lubricant equivalent. A vegetable based oil lubricant may cost twice as much as a mineral-based oil lubricant. Other problems associated with vegetable oils are low thermal and oxidation stabilities, narrow viscosity range and poor flow properties at low temperature. Further, parameters that are important in the context of lubrication such as friction and wear loss may not be good enough.
JP2011140631 discloses a lubrication composition comprising a base oil, a thickening agent, an antioxidant and the coenzyme Q. The base oil may be based on glycerides.
JP2011219690 discloses a lubricant grease composition comprising a triglyceride, an antioxidant and a thickener. The antioxidant is vitamin A or a derivative thereof.
JP2011162606 discloses a lubricating composition comprising a base oil, starch as thickening agent and antioxidants.
DD288169 discloses a lubricating composition comprising water, starch and glycerine.
Tribology International, 69(2014), 39-45 discloses studies of glycerol aqueous solutions as green lubricants, and comparison is made with rapeseed oil. The viscosity of glycerol is lowered to a desired value by addition of water. The friction coefficient of glycerol aqueous solutions having a water content of 30% or less is found to be lower than that of rapeseed oil. However, the wear volume loss, i.e. the volume loss of material that takes place in the contact area of the surface(s) during friction, of glycerol aqueous solutions is higher than that of rapeseed oil. It is stated that glycerol aqueous solutions have great potential as green lubricants and that their lubricating properties are much better than rapeseed oil, especially when the water content is below 20 wt %.
Conveniently, glycerol (which is also known as glycerin, glycerine and propane-1,2,3-triol) is easily available in large quantities and at low cost on the market. This is largely due to the rapidly growing production of bio-diesel where glycerol is formed as a byproduct, and novel applications of glycerol are therefore sought for.
The viscosity and freezing point of aqueous glycerol may be controlled by the amount of water mixed with the glycerol. Thus, by adding water to glycerol viscosity may be lowered to a desired value, while at the same time lowering the freezing point to below that of pure glycerol.
There is thus still a need for improving the properties and the performance of green lubricants.
It is an object of the present disclosure to overcome or at least mitigate some of the problems associated with lubricants such as green lubricants.